The term "biodiversity" was introduced in 1988 by evolutionary biologist E.O. Wilson, one of the leading experts in this field (footnote 1). Discussion of biodiversity has become commonplace in the past several decades. Although scientists are still trying to answer basic questions, including how many species there are on Earth, a broad trend is clear: extinctions are occurring today at an exceptionally high rate, and human activities are a major cause (footnote 2).

Is this a serious problem if millions of species remain? At times the question is posed this way—for example, when the fate of one seemingly obscure organism is at stake—but in fact there are important connections between biodiversity and the properties of ecosystems. For example, a tract of forest land can sustain more plants if it contains significant numbers of organisms that enhance soil quality, such as earthworms and microbes (Fig. 1). As we will see, a change in the status of one species can affect many others in ways that are not always predictable. Healthy ecosystems provide many important services to humans, although these functions are not always recognized or awarded economic value. If biodiversity erodes, we may lose some of these services permanently.

There also is an aesthetic case for maintaining biodiversity. We take it for granted that nature is attractive, but much of the world's appeal is rooted in the contrast between many types of species, whether the setting is a coral reef filled with tropical fish or a forest filled with autumn colors.

This unit explores how scientists define and measure biodiversity, and how biodiversity is distributed around the globe and divided among the various types of organisms. It then discusses factors that are impacting biodiversity, including habitat loss, invasion by alien species, and over-harvesting. Emerging threats to biodiversity from global climate change are addressed in Unit 12, "Earth's Changing Climate," and Unit 13, "Looking Forward: Our Global Experiment."